1. Field of the Invention
The field of the invention includes a photoactive coating composition and methods and materials for making a photoactive coating composition. More particularly, the field of the invention includes a sunscreen composition for use on human skin and methods and materials for formulating a sunscreen composition.
2. Brief Description of Related Technology
It is well known that ultraviolet radiation (light) having a wavelength from about 280 nm or 290 nm to about 320 nm (UV-B) is harmful to human skin, causing burns that are detrimental to the development of a good sun tan. UV-A radiation (about 320 nm to about 400 nm), while producing tanning of the skin, also can cause damage, particularly to very lightly-colored or sensitive skin, leading to reduction of skin elasticity and wrinkles. Therefore, a sunscreen composition for use on human skin preferably includes both a UV-A and a UV-B filter to prevent most of the sunlight within the full range of about 280 nm or 290 nm to about 400 nm from damaging human skin.
Ultraviolet radiation from the sun or artificial sources can also cause harm to coatings containing photoactive substances, such as photoactive pigments and dyes, by breaking down chemical bonds in the structure of a component such as a polymer, a pigment, or a dye. This photodegradation can lead to color fading, loss of gloss, and loss of physical and protective properties of a coating. Photodegradation can take place in several steps which include one or more components of a coating absorbing UV radiation. The absorbed radiation can excite the absorbing molecules and raise them to a higher energy level, which can be very reactive. If the molecule cannot be relaxed, bond cleavage and the formation of free radicals will occur. These free radicals can attack one or more color molecules and/or a polymer backbone and form more free radicals. UV-A and UV-B filters can also be used to absorb UV radiation to protect a pigmented coating.
The UV-B filters that are most widely used in the U.S. in commercial sunscreen compositions are paramethoxycinnamic acid esters, such as 2-ethylhexyl paramethoxycinnamate, commonly referred to as octyl methoxycinnamate or PARSOL MCX, octyl salicylate, and oxybenzone.
The organic UV-A filters most commonly used in commercial sunscreen compositions are the dibenzoylmethane derivatives, particularly 4-(1,1-dimethylethyl)-4xe2x80x2-methoxydibenzoylmethane (also called avobenzone, sold under the brand name PARSOL 1789). Other dibenzoylmethane derivatives described as UV-A filters are disclosed in U.S. Pat. Nos. 4,489,057; 4,387,089 and 4,562,067, the disclosures of which are hereby incorporated herein by reference. It is also well known that the above described UV-A filters, particularly the dibenzoylmethane derivatives, can suffer in rapid photochemical degradation, when used alone or when combined with the above-described most commercially used UV-B filters.
Typically, the above-described UV-B filters are combined with the above-described UV-A filters in a solution with other lipophilic or oily ingredients. This solution of oily ingredients, known to formulators of cosmetic products including sunscreens as the xe2x80x9coil phase,xe2x80x9d is typically, but not necessarily, dispersed with the help of emulsifiers and stabilizers into an aqueous solution composed primarily of water, to make an emulsion which becomes a final cream or lotion form of a sunscreen composition.
Sunscreen performance has been extremely difficult to predict based on the levels of sunscreen active compounds in the formulation, particularly when the formulation includes one or more sunscreen active compounds that suffer from relatively rapid photodegradation, such as avobenzone. Because of this, each formulation has required expensive laboratory testing to determine the UV absorbance, as a function of time (quantity) of exposure of the formulation to UV radiation.
The formulation of a final sunscreen composition has been achieved largely through an iterative trial-and-error process, based on known rules of thumb. A UV filter system (one or more UV-absorbing compounds) is selected to provide UV-A and/or UV-B protection. A solvent system is selected to dissolve the components of the UV filter system. Next, the concentrations of each component in the filter system dissolved in the solvent system are selected in an attempt to achieve a sunscreen having a particular sun protection factor (SPF). If the selected solvent system is not able to completely dissolve the UV filter system at the desired concentrations of UV filter components, then the solvent system must be changed. For example, a greater amount of one or more solvents can be used (effectively lowering the concentration of the UV filter system), or different solvents can be used in the solvent system. The formulated sunscreen composition is then tested in panels of human volunteers, e.g., according to a sunscreen monograph (see, e.g., Title 21 of the U.S. Code of Federal Regulations, Part 351) to measure the SPF of the composition.
In a SPF test, sunscreen compositions applied to the skin receive doses of UV energy simulating sun exposure. For example, for a product to be labeled as SPF 30 in the U.S., it must prevent sunburn until a UV dose equivalent to 30 times the minimal erythema dose (MED; 1 MED is about 21 mJoule/cm2) is received. This dose is approximately equivalent to a full day of summer sun exposure.
If the sunscreen composition does not achieve the desired SPF, then the composition must be reformulated, typically by adding additional quantities of UV filters. Again, if the solvent system is not able to completely dissolve the UV filter system at the desired concentrations of UV filter components, then the solvent system must be changed again. The reformulated sunscreen composition containing a higher concentration of UV filters and, potentially, a different solvent system, is then re-tested to gauge SPF.
This iterative trial-and-error process is further complicated by the photodecay of the photoactive compounds in the sunscreen composition, such as UV filters. Sunscreen compositions containing rapidly-degrading photoactive compounds (e.g., avobenzone) degrade in rapid exponential fashion when exposed to UV radiation, but start out with SPF values several-fold higher than their values after dosing with UV radiation. Thus, the SPF rating after testing (e.g., after 30 MED) are the cumulative SPF values over the period of UV irradiation while one or more compounds may be degrading and losing effectiveness, further complicating the ability to formulate a sunscreen composition with a desired effectiveness.
One aspect of the invention is a method of preparing a sunscreen composition including the step of controlling the polarity of a solvent system in the composition to control the rate of photodecay of a filter system in the composition.
Another aspect of the invention is a method of formulating a sunscreen composition, including the steps of selecting a filter system; preparing parallel mixtures of the filter system in a plurality of analytical solvent systems, all of the mixtures having substantially the same concentration of filter system; determining the polarity of each of the mixtures; determining a rate constant of photodecay of each of the mixtures, or sunscreen compositions including the mixtures; selecting a final solvent system based on its polarity; and mixing the filter system and the final solvent system.
Yet another aspect of the invention is a composition including a filter system and a solvent system, wherein a mixture of the filter system and the solvent system in the ratio present in the composition has a high polarity, for example, a composition having a high polarity may have a dielectric constant at least about 7.
Still another aspect of the invention is a sunscreen composition including a filter system and a solvent system, wherein the solvent system includes a amide containing a C4-C40 hydrocarbon.
Another aspect of the invention is sunscreen composition including a filter system and a solvent system, wherein the solvent system includes a diester of malic acid, wherein the malic acid is esterified with a C4-C30 hydrocarbon.
Yet another aspect of the invention is a sunscreen composition including a filter system and a solvent system, wherein the rate constant of photodecay of the filter system is about 200% or less of the theoretical minimum rate constant of photodecay.
Further aspects of the invention may become apparent to those skilled in the art from a review of the following detailed description, taken in conjunction with the appended claims. While the invention is susceptible of embodiments in various forms, described hereinafter are specific embodiments of the invention with the understanding that the disclosure is illustrative, and is not intended to limit the invention to the specific embodiments described herein.